Mechanical face seals provided with annular working ring made of cemented carbide are known and their advantages in comparison with mechanical solid carbide seals are explained for example in U.S. Pat. No. 4,280,841 assigned to Nippon Tungsten Co.
In this patent is described manufacturing of mechanical seal provided with a cemented carbide hardened layer or ring which is firmly bound to the seal substrate. According to one of the embodiments disclosed in this patent powder of tungsten carbide mixed with an addition of 6.5 percent of Co is placed within a groove formed in stainless steel substrate and is compressed therein to produce a green compact. The compact is then presintered by heating in vacuum to obtain carbide layer with the thickness 1.3 mm. Then a paste which is a mixture of Ni--P alloy is coated or sprayed onto the presintered compact and finally the presintered carbide compact is heated in a non-oxidizing atmosphere to obtain cemented carbide hardened layer strongly bonded with the substrate by virtue of diffusion-bonding effect.
The disadvantage of this method lies in the fact that use of low melting Ni--P alloy does not allow obtaining cemented carbide working ring with sufficient hardness. The function of Ni--P contained in the upper layer is merely to infiltrate into the bulk of the tungsten carbide and to provide for soldering with the substrate. The reported hardness in the '841 patent of the working ring lies between H.sub.v 720-850 which is almost two times less than that of the solid carbide. Despite it is stated in the patent that the above hardness showed improved wear resistance one can assume that it might be still insufficient for many heavy duty applications. Moreover, due to relatively low hardness one can expect that mechanical end seal rings manufactured by this method have high coefficient of friction and are prone to seizure at relatively low loads.
The further disadvantage of the known method is associated with the fact that the thickness of the obtained working ring is limited to only 1.3 mm which shortens the service life of a seal provided with such a ring.
In GB1290980 is disclosed method of obtaining a wear-resistant surface on a steel part. This method comprises coating of at least the bottom of the groove made in the part by a copper layer, placing thereon a layer of powdered tungsten carbide, pressing this layer within the groove, superimposing an upper layer of copper powder on the layer of tungsten carbide, pressing the copper layer and then heating the whole part in a neutral atmosphere which is sufficient to melt the copper. The copper residing in the upper layer melts, impregnates the bulk of tungsten carbide portion and binds thereof, while the copper layer placed on the bottom of the groove provides reliable soldering of the bulk tungsten carbide portion to the substrate.
Unfortunately since copper is very prone to corrosion in presence of many industrial gases and liquids, especially H.sub.2 S, organic acids, sulfuric acid, ammonia, sodium hydroxide, distilled water or natural gas the face seals manufactured by the above method are not suitable for use in those industrial applications where such gases or liquids might be expected.
Furthermore, the necessity in coating the groove walls by the copper layer renders this method technologically complicated.
In RU2021078 is described method of production of wear-resistant layer on working end surface of a face seal. This method involves filling circular groove made in a steel based substrate by a powdered tungsten carbide, pressing thereof within the groove, placing on the green compact of a layer of powdered Cu--P alloy, pressing this layer and then heating the substrate in vacuum until the upper layer melts and impregnates the tungsten carbide portion. It is mentioned that Cu--P effects reliable soldering between the substrate and tungsten carbide and thus eliminates the necessity in preliminary coating the groove by a copper layer.
Unfortunately since this method also utilizes copper it is inevitably associated with the same disadvantage, i.e. the seals manufactured according to this method are insufficiently chemically resistant in the presence of industrial or natural fluids.
In conclusion it should be emphasized that despite the fact that different methods for manufacturing of mechanical end seals employing working cemented carbide ring have been devised there is still a need for a new method ensuring producing of such face seals with improved performances comparable with the seals made of solid carbide, but being cheaper than solid carbide face seals.